The present invention relates to an optical device for proximity coupling between two waveguides integrated in a substrate, and more particularly, to such a device which is characterized by low loss, broadbanded operation and which possesses acceptable cutoff wavelength characteristics.
Cost reductions in optical networks can be obtained by sharing the fiber among multiple subscribers. The ion-exchange technique, for example, has proved to be a promising technology for producing 1.times.N splitters with output ports numbering as high as 16. Such devices, when formed as Y junctions, provide excellent achromaticity and uniformity at low loss.
The ability to provide a second input to a system is becoming more desirable for various reasons such as (a) the multiplexing of two different signals; (b) redundancy in different paths to reach a splitting point; (c) flexibility in the future deployment of the network; and (d) providing a network testing entry point. Junctions which can provide a second input, a 2.times.2 junction, for example, are more difficult to realize than a 1.times.2 junction, when using planar Y junction technology. The combination of two Y junctions, one to provide a second input to a 1.times.N splitter, results in a 3 dB additional loss; such high loss is unacceptable.
Therefore, interferometric devices have been employed for combining signals in integrated circuits. FIG. 1 shows a symmetrical waveguide coupler 10 which functions as a wavelength division multiplexer (WDM) for the combining/separating of two signals of different wavelengths. It comprises two straight parallel waveguides 11 and 12 and curved approach segments 13, 14, 15 and 16, the ends of which are referred to as input/output ports. The ports are separated by a distance fixed by the diameter of coated optical fibers 19-22 which are attached to these ports by a technique of "pigtailing", for example.
A broadbanded optical waveguide coupler is disclosed in the publication, A. Takagi et al. "Broadband Silica-Based Optical Waveguide Coupler with Asymmetric Structure", Electronics Letters, 18 Jan. 1990, Vol 26, No. 2, pp. 132-133. The device is made broadbanded by forming the two paths such that they have different propagation constants in the coupling region. Referring to FIG. 1, propagation constants are made different by changing the width of one of the waveguide paths. Straight waveguide 11 and segments 13 and 15 remain the same width as they were in the WDM coupler, but waveguide 12' is narrower than waveguide 11, approach segments 14 and 16 being tapered from standard width at the input and output ports to the narrower width at waveguide 12'.
In a .DELTA..beta. proximity coupler the power transferred from one waveguide to the other is given by ##EQU1## where L is the length of the interaction, C is the coupling constant, and F depends upon .DELTA..beta., the difference between the propagation constants .beta..sub.1 and .beta..sub.2 of the two waveguides, and is given by ##EQU2## In order to make a 3 dB coupler, it can be shown that the wavelength response of the proximity coupler is flattened to a maximum extent when F.sup.2 equals 1/2 and L is chosen in such a way that CL/F equals (2n+1).PI./2. Incomplete power transfer occurs, whereby output power as a function of wavelength is flatter.
The .DELTA..beta. coupler of the Takagi et al. publication is formed by a combination of flame hydrolysis deposition of SiO.sub.2 on Si, photolithography, and reactive ion etching. A relatively small excess loss is induced in the curved portions of the narrowed approach segments 14' and 16' because of their reduced diameter.
The ion-exchange process is different from flame hydrolysis or other planar techniques in that it is impossible to reduce the width of an ion-exchange waveguide path without reducing its maximum index of refraction. Therefore the effective index of the narrower path is reduced by both (a) the narrowing of the path width and (b) the diminution of the refractive index. When ion-exchange technology is used to form the waveguide paths in a .DELTA..beta. proximity coupler, both effects (a) and (b) will increase the effective index difference between the two paths (and thus the .DELTA..beta.), and both effects will also increase the losses in the bend approach region.